The authors have studied the optical input interface for Nb single-flux-quantum (SFQ) circuits and demonstrated its operation using photomixing technique. Two tunable lasers of C band were used for photomixing. We adopted a multistage toggle flip-flop (TFF) circuit as the main part of the test circuit. As the first trial, we used lasers with wavelength difference of 0.044nm at around 1550nm, which corresponds to the laser beat frequency of 5.5GHz. The output signals from the 13- and 14-stage TFFs were observed and it showed the input SFQ frequency of 5.5GHz. Thus, the authors demonstrated the conversion of the optical signal to SFQ signal and succeeded in driving the SFQ circuit.